(1.) Field of the Invention The present invention relates to an image-forming apparatus employing a so-called reversal developing system, and more specifically to an image-forming apparatus of a reversal developing system preventing the occurrence of image spots.
(2.) Description of the Prior Art
An image-forming apparatus employing the reversal developing system has heretofore been known. In this reversal developing system, the photosensitive material is uniformly charged to a positive or negative polarity, the image is exposed to a laser beam or a like beam, and electrostatic latent image is formed on a portion irradiated with light that corresponds to the document image maintaining a residual potential of 0 V to 100 V attenuated by the light. Then, the toner charged to the same polarity as the charging polarity of the photosensitive material is brought into contact with the photosensitive material to effect developing, and the toner adhered to the surface of the photosensitive material on a portion having the potential of 0 V to 100 V is transferred onto a transfer material such as a paper to form the image.
According to the above-mentioned method of forming image based on the reversal developing system, the toner image formed on the surface of the photosensitive material is transferred by applying a DC voltage of a polarity opposite to the polarity of the charge of the toner image to the transfer roller. During the step of transfer, therefore, the surface of the photosensitive material is likely to be charged into an opposite polarity. Besides, the discharge is not effectively carried out and the subsequent image cannot be effectively formed.
In order to solve the above-mentioned defect of the image-forming apparatus employing the reversal developing system, Japanese Laid-Open Patent Publication No. 7086/1989 proposes means in which a DC voltage applied to the transfer roller is set to be lower than a charge start voltage at which the photosensitive material starts electrically charged.
According to the above-mentioned means of the prior art, however, the transfer roller is impressed with a DC voltage which is lower than the charge start voltage. Therefore, the transfer becomes poor due to a drop in the transfer efficiency and leaves problems that must be solved for obtaining good image.
In the image-forming apparatus of the reversal developing system, when a voltage higher than the charge start voltage is applied to the transfer roller, on the other hand, there develops a charge of a polarity opposite to that of the initial charge of the photosensitive material. The potential of the charge of this opposite polarity cannot be removed through the step of discharge effected prior to beginning the step of electrophotography. During the step of main charging, therefore, there develop portions having low charge potentials due to offset in the potential, resulting in the occurrence of spots in the charge potential, i.e., image spots. The image spots appear to be offensive even on images described with lines and become conspicuous particularly on half-tone images.